28th International Cosmic Ray Conference

155

A Study of Nuclear Composition of Primary Cosmic Rays
above 100 TeV
India-Japan GRAPES Collaboration:
H. Tanaka,2 S.K. Gupta,1 Y. Hayashi,2 Y. Ishida,2 N. Ito,2 Atul Jain,1A.V. John,1 S. Kawakami,2 H. Kojima,3 T. Matsuyama,2 D.K. Mohanty,1P.K. Mohanty,1 S.D. Morris,1 T. Nonaka,2 A. Oshima,2 K.C. Ravindran,1M. Sasano,2 K. Sivaprasad,1 B.V. Sreekantan,1 S. Tamaki,2 S.C. Tonwar,1K. Viswanathan1 and T. Yoshikoshi2(1) Tata Institute of Fundamental Research, Mumbai 400005, India(2) Graduate School of Science, Osaka City University, Osaka 558-8585, Japan(3) Nagoya Women’s University, Nagoya 467-8610, JapanAbstract
The GRAPES-3 air shower array is located at Ooty (800g/cm2) in southern
India and consists of 256 scintillation detectors (1m2 each) and 16 muon detectors(35m2 each). In order to study the nuclear composition of primary cosmic rays
around the knee energy, we investigate the relation between muon multiplicity
and shower size using observations from this experiment during the period, 2000–2001 with ∼ 106 showers having Ne > 105 . These results are compared withMonte Carlo simulations (CORSIKA v6.02 with QGSJET model) to obtain thevariation of the average mass number with primary energy. We have carried out
detailed simulations for ﬁve types of primary nucleus (Proton, He, N, Al and Fe)
for comparison with observations. We present here the details of the results.
1.

Introduction

Our main motivation is to understand physical mechanism of steepeningof the primary cosmic ray energy spectrum around 1015 to 1016 eV , what is calledknee. Though a few observations have been reported about the variation of thenuclear composition in this range recently, there is disagreement among them.
Since it contains the information of their origin and acceleration, it is importantto establish the nuclear composition of primary cosmic rays.
It is well-known that the number of muons in extensive air showers has agood correlation with mass number of its primary cosmic rays. We have installedvery large area (∼ 560m2 ) muon detectors to observe number of muons with highstatistics.

cpp. 155–158 2003
by Universal Academy Press, Inc.

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2.

Experiment

The GRAPES-3 air shower array is located at Ooty in southern India(N 11.4◦ , E 76.7◦ ) at an altitude of 2, 200m above sea level, an atmosphericdepth of 800g/cm2 . It consists of 256 scintillation detectors (SDs) to measure
the electron component and 16 units of proportional counters to detect muons inshowers.
Each of these SDs contains 1m2 area, 5cm thick plastic scintillator. They
are placed in hexagonal shape with 8m separations as shown in ﬁg.1. Each SD
can measure both density and arrival time of charged particles. Trigger rate isaround 14Hz with any 10 ﬁres out of 120 SDs. We determine the shower size andarrival direction for individual showers.
We have large area muon track detectors. Their element is the 6m longproportional counter with a cross section of 10cm by 10cm. 58 counters areplaced side by side on a concrete platform and covered with 2m thick concreteslabs. Four layers of counters are arranged in crossed conﬁguration to identify the
track of individual muon. Each unit has 35m2 detector area, and total 560m2 arecovered. Threshold energy of vertical through muons is about 1GeV . Their data
are recorded with trigger from shower array described above.
3.

Analysis

We carried out analysis of showers between 2000 March and 2001 December, the eﬀective running time is about 560 days. There are ∼ 106 showers having
Ne > 105 whose core locations fall inside a hexagon shown in ﬁg.1.
Detailed analysis has been done for showers that satisfy the following conditions• More than 10 SDs ﬁring out of 120 SDs inner array.
• Core location should be inside an area as shown in ﬁg.1.
• Zenith angle of the air shower should be less than 25◦ .
Observed lateral density distribution is ﬁtted with NKG function to estimate shower core location R, size Ne and age parameter s. We adopted 103m asMolière unit rm .
The followings are the conditions of MC simulation.
• CORSIKA V6.02 with QGSJET model.
• Threshold of primary energy is 10T eV for proton and 80T eV for iron. Thesethresholds are kept so small that they should not give any biases for production of Ne > 104.5 showers.

157Fig. 1. Accepted core location. Triangle marks mean SD arrangement
and square ones mean muon detectors. For this analysis, core location
should be in yellow (or gray) area,
which is inside a hexagon and 80m
away from muon detectors.

Fig. 2. Relations between Shower
Size and Primary Energy with MC.

• Energy cut-oﬀs are 1MeV for electron and 1GeV for muon at the observation height.
• The slope of the primary energy is −2.7. (−3.15 above 3P eV for Proton)
Each simulated shower is randomly distributed within an area of 100m radiusfrom the center of the array and we adopted the showers which satisfy the same
experimental trigger condition.
In order to reject the accidental muons, we have selected the muon tracks
which agree with EAS direction.
4.

Results and Conclusions

Fig.3. shows variation of number of muons with air shower size and corresponding MC results. This Ne – Nµ relation features the primary nuclear composition. There is no signiﬁcant change in their slope within this size range.
Fig.4. shows muon multiplicity distribution in 105.4 ≤ Ne < 105.6 . Eachnuclear composition has characteristic multiplicity distribution. As one can see
from ﬁg.4., major contribution arises from Proton primary below muon multiplicity of 10. Even multiplicity below 20 we need not to consider the contribution fromother nucleus than Proton and Helium. It means we can estimate the abundanceof those two nucleus in the primary nuclear composition using this multiplicity
distribution (MC’s). It will enable us to give some constraint to composition

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Fig. 3. Average number of detectedmuon with shower size. Observation is shown in black and compared with MC results.

Fig. 4. Distribution of detectedmuon number with size. Observation is shown in black and others
are MC results.

model of galactic cosmic rays. Estimation of average mass number for these energy range is still underway and will be given in the conference.
GRAPES-3 has a potential to decide variation of cosmic ray composition
with large area muon detectors.
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